1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus having a sheet feeding apparatus for separating and feeding sheets by blowing an air to the sheets.
2. Description of the Related Art
Hitherto, an image forming apparatuses such as printer, copying apparatus, or the like has a sheet feeding apparatus for feeding sheets one by one to an image forming unit. In such a sheet feeding apparatus, in order to certainly separate and feed the sheets one by one, various sheet separating systems have been used hitherto.
As such a sheet separating system, for example, there is a retard separating system in which a retard roller (separating roller) driven in the direction opposite to the sheet conveying direction is come into pressure contact with a feed roller at a predetermined torque, thereby preventing double feeding. There is also a separating pad system in which a frictional member is come into pressure contact with the feed roller, thereby preventing double feeding.
In the case of separating and feeding the sheets by such a sheet separating system, for example, in the case of the retard separating system, by optimizing one of a return torque and a pressure-contact force of a retard roller in consideration of a frictional force of the sheet to be fed, the sheets can be certainly separated one by one.
In recent years, in association with the diversification of a variety of sheets (record media), there has been increasing a demand for forming images onto sheets such as super thick paper, OHP sheet, art film, and also coating paper obtained by executing a coating process to the surface of a sheet in order to obtain a whiteness index and a gloss in order to meet a market requirement to form a color image, and the like.
However, in the conventional sheet feeding apparatus, in the case of continuously feeding such sheets, for example, in the case of feeding the super thick paper, its own weight becomes a conveyance resistance, so that the sheets cannot be picked up but a jam occurs.
In the case of a sheet such as OHP sheet or art film made of a resin material which is liable to be charged, particularly, upon feeding in a low humidity environment, the sheets rub mutually, so that the sheet surfaces are gradually charged. Since the sheets themselves are adhered by such charging of the sheet surfaces, the sheets cannot be picked up and the double feeding occurs.
In the case of the sheet such as coating paper whose surface has been coated with a paint or the like, particularly, in the case where the sheets have been stacked in a high humidity environment, such sheets have a nature in which the sheets themselves are attracted. Therefore, in the conventional sheet separating system, the sheets cannot be picked up and the double feeding occurs.
This is because in the case of such special sheets, although a frictional force itself of the sheets is equal or smaller than that of plain paper mentioned above, an attracting force obtained by the frictional charging in the low humidity environment and an attracting force between the sheets of coating paper in the high humidity environment are remarkably larger than a frictional force between the sheets of plain paper. On the other hand, according to the conventional sheet separating system, since only the frictional force between the sheets is considered, the sheets attracted by such a strong force cannot be fully separated.
Therefore, among the conventional sheet feeding apparatuses, in order to release the adsorption of the sheets attracted by the very strong attracting force, there is an apparatus in which the sheets are fed by an air sheet feeding system using a sheet separating system in which the air is blown to side portions of the sheets, and the adsorption between the sheets is eliminated by the air.
In the case of the sheet feeding apparatus for feeding the sheets by the air sheet feeding system, when the sheets are fed, the air is blown from the side portion of a sheet bundle by a pre-separating air loosening unit, thereby preliminarily loosening the sheets and eliminating the adsorption between the sheets.
Subsequently, the sheets which have previously been loosened are attracted to an attracting and conveying belt in order from the top sheet. And the separating air is blown to the sheets from a separating air unit provided on a downstream side of the belt, thereby separating the sheets one by one. The sheet feeding apparatus using such an air sheet feeding system is used in a print business world and partial copying apparatuses because even the sheets having the large attracting force can be separated and fed.
In such an air sheet feeding system, particularly, in order to reduce the attracting force of the sheets such as coating paper or the like in the high humidity environment, a method of dehumidifying the sheets by heating the blowout air by a heater has been proposed. Such a technique has been disclosed in Japanese Patent Application Laid-Open No. 2001-048366.
FIG. 6 is a diagram illustrating such a construction of the conventional sheet feeding apparatus in which the sheets are dehumidified by a heated blowout air. In FIG. 6, a sheet feeding apparatus 100 has an attracting and conveying belt 101. The attracting and conveying belt 101 can circle in the sheet feeding direction. The belt 101 attracts a top sheet St among sheets S stacked on a tray (not shown) and conveys the sheet St to the downstream.
A sucking duct 103 for activating a negative pressure is provided inside of the attracting and conveying belt 101. The negative pressure is activated in order to suck the top sheet St by a sucking air which is caused by a sucking fan 102. A pre-separating nozzle 110 for floating and loosening the sheets S is provided under the downstream side in the rotating direction of the attracting and conveying belt 101. The pre-separating nozzle 110 blows a loosening air to the sheets S stacked on the tray from their front edge side surfaces, thereby allowing the sheets S to be floated and loosened. In order to prevent the double feeding, a separating nozzle 109 for blowing a separating air so as to peel off a next sheet Sn from the top sheet St attracted onto the attracting and conveying belt 101 is also provided.
In FIG. 6, a fan 106 sucks the air into the duct 107. The air sucked by the fan 106 is guided from the duct 107 to a separating air duct 104 and a pre-separating air duct 105. An air heater 108 is provided in the duct in order to heat the air sucked by the fan 106.
In FIG. 6, an air As is blown out from the separating nozzle 109 and an air Ap is blown out from the pre-separating nozzle 110. The attracting and conveying belt 101 is suspended between a driving roller 101a and a driven roller 101b, so that the attracting and conveying belt 101 is circled by those rollers.
In the sheet feeding apparatus 100 mentioned above, after the air sucked into the duct 107 by the fan 106 passed through the heater 108, the air is separated into the separating air duct 104 and the pre-separating air duct 105. Thereafter, the air is blown to the outside as one of the separating air As from the separating nozzle 109 and the pre-separating air Ap from the pre-separating nozzle 110.
The pre-separating nozzle 110 faces the edge portions of the sheets S. By blowing the pre-separating air Ap to the sheet edge portions, the stacked sheets S can be loosened by the air and the adsorption between the sheets can be eliminated. The separating air As is blown toward the attracting and conveying belt 101 from the separating nozzle 109.
Subsequently, when driving the sucking fan 102, the top sheet St stacked in the top position of the sheets S in the state where they have been loosened is sucked to the attracting and conveying belt 101 by the sucking force of the sucking fan 102. Thereafter, by driving the driving roller 101a, the attracting and conveying belt 101 is circled in the state where the top sheet St has been attracted to the belt, so that the top sheet St is conveyed. When the top sheet St is conveyed in this manner, the next sheet Sn just under the top sheet St is separated by the separating air As from the separating nozzle 109.
The air heater 108 is controlled based on external temperature and humidity detected by an environment sensor (not shown). Specifically speaking, the air heater 108 is turned on in the high temperature and high humidity environment and is turned off in the low temperature and low humidity environment.
By turning on the air heater 108 in the high temperature and high humidity environment as mentioned above, the pre-separating air Ap and the separating air As can be heated, so that the sheets S can be dehumidified. Thus, the attracting force between the sheets due to the moisture adsorption can be reduced and the sheets can be certainly separated. There is also an apparatus in which in the case of particularly the thick sheet, by blowing a superheated steam to the sheets, the sheets S are dehumidified. Such a technique has been disclosed in Japanese Patent Application Laid-Open No. 2002-333275.
However, in such conventional sheet feeding apparatus and image forming apparatus as mentioned above, when the sheets are separated by blowing the air, particularly, in the low humidity environment, there is a case where portions of the stacked sheets S near the pre-separating nozzle 110 are partially dried by the pre-separating air Ap. If the sheets S are partially dried as mentioned above, a variation occurs in surface resistance values in the sheet surfaces.
Particularly, in the case where the image forming apparatus of an electrophotographic system for transferring a toner image onto the sheet by using static electricity, transfer performance is largely influenced by the surface resistance value of the sheet surface. Therefore, if the sheets S are partially dried and the variation occurs in the surface resistance values in the sheet surfaces as mentioned above, a defective transfer occurs only in the dried portions, so that an image deterioration occurs and image quality deteriorates.